Several magnetic mixing devices for fluid streams have been described. U.S. Pat. Nos. 3,219,318 to Herschler, 3,680,843 to Lu et al, 3,689,033 to Holmstrom et al, 3,907,258 to Spaziani, and 4,054,270 to Gugger et al disclose the location of permanent magnets in a fluid stream within a conduit for agitation by an external magnetic field which reverses in polarity. This magnetic field is typically provided by a single electromagnet having its poles to opposite sides of the condut. Another device, described in U.S. Pat. No. 3,763,873 to Saunders, uses a magnetically susceptible tubular reed which also serves as an inlet tube and is therefore fixed at one end. The free end is agitated and indeed vibrated by a spinning permanent magnet. Other prior magnetic mixing configurations are disclosed in U.S. Pat. Nos. 2,999,673 to Kessler, 3,784,170 to Peterson et al, 3,793,886 to Rosenwald and 3,995,835 to Cichy et al.
Many flow based methods for chemical analysis, for example high pressure liquid chromatography and discontinuous flow analysis (described in the present applicant's international patent application No. PCT/AU86/00323), require two or more fluid streams to be mixed uniformly transverse to the direction of flow but so that any time variation in the ratio of comonents of the streams is maintained without appreciable broadening in the direction of the flow. In this respect, the arrangements of the prior art are severely limited, particularly where small bore tubing, for example less than 1 mm internal diameter, is involved. Minature magnets suitable for insertion in such tubing are not commonly available and must be matched in size to the external magnetic field to provide effective coupling. Further, the turbulence arising from the Y and T fluid junctions of the aforementioned U.S. Pat. Nos. 3,689,033 and 3,907,258 respectively will cause disruption of the ratio of components prior to mixing. The violent agitation at the point of confluence in U.S. Pat. No. 3,763,873 will cause considerable broadening, as will the movement of the "plurality of permanent magnets" of U.S. Pat. No. 3,219,318. U.S Pat. No. 3,763,873 does not use a permanent magnet for agitation, but the mixing action is restricted to a very small part of the flow line in the vicinity of the end of the tubular reed and is not considered effective for high flow rates.